1. Field of the Invention
The present invention relates to an apparatus, system, and method for multiplexing signals from a plurality of channels and transmitting them as an optical signal.
2. Description of the Related Art
FIG. 4 shows an example of a time-division multiplexing procedure in which electrical signals (binary signals) from four channels are combined into an optical signal having four times the bit rate of the individual channels. In this example, the electrical signal in each channel is converted to an optical pulse train having a pulse width sufficiently narrow to allow a quadrupling of the bit rate. The four optical pulse trains are combined with different time delays in an optical coupler or similar device, and sent on a transmission line such as an optical fiber.
In optical time-division transmission apparatus of this type, the average optical power of the optical signal being transmitted may vary due to loss of signal on one or more channels, or to new signal supply in a channel that had been lost or idle. FIG. 5 shows an example in which the average optical power of a signal carrying four channels (Ch1-4) is reduced by loss of the third channel (Ch3). The average optical power is reduced because the average number of optical pulses per unit time is reduced, reducing the average duty cycle of the optical signal.
A typical optical signal transmission system including optical time-division transmission apparatus of the above type carries out control to maintain the average optical power of the optical signal at a constant value in order to maintain a constant transmission quality. If the signals on one or more channels are lost during optical signal transmission, however, or if a new signal is supplied in a channel that had been lost or idle, this conventional control scheme may cause problems. An example is shown in FIG. 6. During the transmission of signals on channels 1 to 4, the signals on channel 2 and channel 4 are lost, reducing the average number of optical pulses per unit time, so the power (amplitude) of the individual optical pulses is increased in order to maintain a constant average optical power. Then the signal on channel 3 is also lost, and the power of the individual optical pulse is increased still further. These variations in the power of the individual optical pulses can have adverse effects on the quality of signal transmission, by causing a deterioration of the optical S/N ratio or giving rise to non-linear optical effects.